Are you thinking of something like:
HAL, add this / that to the diary.?
Most new conversational topics are automatically
added to Hal's database but I imagine that a table / routine
could be constructed to place a statement into such a
diary of sorts.
I also haven't seen the previous statement routine used for
Hal 6. Hal 5 had a "What did you just say?" script that allowed
Hal to recall previous topics.
Always room for improvement.